Α-diimine Palladium Research Articles

Double stimulus-responsive palladium catalysts for ethylene polymerization and copolymerization

The external stimulus response strategy has been evolved rapidly in the field of olefin polymerization. In this work, we modularly synthesized three types of double stimulus responsive α-diimine palladium catalysts, combining redox regulation and other regulation together, such as light, Lewis acid and alkali cations. The catalytic activities and the molecular weight of polyethylene products can be regulated for 4 times in ethylene polymerization. These palladium complexes were also used for the copolymerization reaction of ethylene and polar monomers, such as methyl 10-undecylenate and methyl acrylate, effectively regulating the catalytic activities, the molecular weight and polar monomer incorporation of the prepared copolymers. The research on these dual-regulated palladium complexes makes full use of prepared catalysts and provides new inspirations for regulating olefin polymerization.

Ligand pre-inserted α-diimine palladium catalysts and mechanism studies in olefin polymerization

In this work, ligand pre-inserted α-diimine palladium complexes were synthesized to catalyze the ethylene polymerization and copolymerization with polar monomers.

α-Diimine-based conjugated microporous polymers as heterogeneous ligands for highly efficient palladium-catalyzed direct C–H arylations

α-Diimine palladium embedded conjugated microporous polymers (DIM-Pd-CMPs) as highly efficient heterogeneous catalysts for direct C–H arylations of heteroarenes with heteroaryl bromides.

Concerted flexible and steric strategy in α-diimine nickel and palladium mediated insertion (Co-)Polymerization

To address the issues of poor thermostability of catalyst and low polymer molecular weight resulted from chain transfer at high temperature in olefin polymerization, versatile strategies have been utilized in the benchmark α-diimine nickel and palladium catalysts. In this contribution, a concerted flexible and steric strategy was highlighted in the α-diimine nickel and palladium catalysts. Compared to the seminal Brookhart catalyst, these newly generated nickel catalysts exhibited improved thermostability over a wide range of temperature from 30 °C to 130 °C, showed higher catalytic activities, and produced significantly higher (4.7–5.8 times) molecular weight polymers in ethylene polymerization, which are highly branched polyethylenes (76–110/1000C, SR = 72 %). Notably, high molecular weight polyethylene (up to 219 kg mol−1) was even afforded at 130 °C, which is extremely challenging when the flexible substituent is used in α-diimine system. The corresponding palladium analogues by using the same strategy further produced high molecular weight ethylene-methyl acrylate copolymers (up to 137 kg mol−1) at the difficult 70 °C in the copolymerization of ethylene and methyl acrylate. The incorporation of methyl acrylate could be tuned (0.6–2.9 mol%), where methyl acrylate units distribute both in the main chain and at the end of branches (15 %:85 %).

A DFT Study of the Copolymerization of Methyl Vinyl Sulfone and Ethylene Catalyzed by Phosphine–Sulfonate and α-Diimine Palladium Complexes

Density functional theory (DFT) calculations were comparatively carried out to reveal the origins of different catalytic performances from phosphine–benzene sulfonate (A, [{P^O}PdMe(L)] (P^O = Κ2-P,O-Ar2PC6H4SO3 with Ar = 2-MeOC6H4)) and α-diimine (B, [{N^N}PdMe(Cl)] (N^N = (ArN=C(Me)-C(Me)=NAr) with Ar = 2,6-iPr2C6H3)) palladium complexes toward the copolymerization of ethylene and methyl vinyl sulfone (MVS). Having achieved agreement between theory and experiment, it was found that the favorable 2,1-selective insertion of MVS into phosphine–sulfonate palladium complex A was due to there being less structural deformations in the catalyst and monomer. Both the MVS and ethylene insertions were calculated, and the former was found to be more favorable for chain initiation and chain propagation. In the case of α-diimine palladium system B, the resulting product of the first MVS insertion was quite stable, and the stronger O-backbiting interaction hampered the insertion of the incoming ethylene molecule. These computational results are expected to provide some hints for the design of transition metal copolymerization catalysts.

Experimental and theoretical insights into palladium-mediated polymerization of para-N,N-disubstituted aminostyrene.

Experimental and theoretical insights into polymerization of para-N,N-disubstituted aminostyrene monomers (St-4-NR2, R = Me, Et, Ph) using cationic α-diimine palladium complexes have been initially reported. The effects of the catalyst structure and monomer substituent were studied systematically. Polymerization turnover frequency (TOF) was shown to decrease in the order of monomer substituents Me > Et > Ph, whereas the molecular weight of the produced polymers showed an opposite trend (Me < Et < Ph). Methanol-mediated polymerization of para-N,N-dimethylaminostyrene (DMAS), along with polymer chain-end analysis, and palladium intermediate isolation proved that palladium-initiated DMAS polymerization obeyed a cationic mechanism. Comprehensive theoretical calculations further revealed that the carbocation was generated from the insertion of DMAS into the palladium center rather than the polarization of the methyl palladium intermediate with a coordinated DMAS. The produced amine-functionalized amorphous polystyrenes have low stereoregularity and exhibit good hydrophilic properties. The poly(para-N,N-disphenylaminostyrene) is a luminescent polymer and shows fluorescence properties, rendering this material a promising candidate for versatile potential applications.

Unsymmetrical Strategy on α-Diimine Nickel and Palladium Mediated Ethylene (Co)Polymerizations.

Among various catalyst design strategies used in the α-diimine nickel(II) and palladium(II) catalyst systems, the unsymmetrical strategy is an effective and widely utilized method. In this contribution, unsymmetrical nickel and palladium α-diimine catalysts (Ipty/iPr-Ni and Ipty/iPr-Pd) derived from the dibenzobarrelene backbone were constructed via the combination of pentiptycenyl and diisopropylphenyl substituents, and investigated toward ethylene (co)polymerization. Both of these catalysts were capable of polymerizing ethylene in a broad temperature range of 0-120 °C, in which Ipty/iPr-Ni could maintain activity in the level of 106 g mol-1 h-1 even at 120 °C. The branching densities of polyethylenes generated by both nickel and palladium catalysts could be modulated by the reaction temperature. Compared with symmetrical Ipty-Ni and iPr-Ni, Ipty/iPr-Ni exhibited the highest activity, the highest polymer molecular weight, and the lowest branching density. In addition, Ipty/iPr-Pd could produce copolymers of ethylene and methyl acrylate, with the polar monomer incorporating both on the main chain and the terminal of branches. Remarkably, the ratio of the in-chain and end-chain polar monomer incorporations could be modulated by varying the temperature.

A comprehensive picture on chain walking olefin polymerization

α-Diimine nickel and palladium catalyzed α-olefin polymerization could produce polyolefins with complicated branching structures due to the chain walking feature. The thorough understanding of the polymer microstructure could provide a general picture of monomer enchainment including the insertion selectivity and chain walking pathways. By utilizing α-diimine nickel (DBB-Ipty-Ni) and palladium (DBB-Ipty-Pd) catalysts reported previously by us that show an excellent selectivity in ethylene polymerization, homopolymerizations of linear α-olefin such as 1-pentene, 1-hexene and 1-octene, and non-linear α-olefin 4-methyl-1-pentene (4M1P) were conducted. In-depth studies of the polyolefin microstructures revealed the enchainment of α-olefin monomers, in which the enchainment of 4M1P indicated a chain-end control on the insertion regio-selectivity. Moreover, internal olefins including cis-2-hexene and cis-3-hexene, and non-conjugated diolefin 1,7-octadiene were successfully polymerized by the α-diimine palladium catalyst. It is worth noting that the isomerization of internal olefins to 1-alkenes in polymerization was found. Chain walking cyclopolymerization of 1,7-octadiene occurred to produce an alternating cyclic copolymer bearing cis-five- and cis-six-membered rings on the polymer backbone. A rational enchainment mechanism was proposed to explain the formation of the special alternating cyclic chain structure. In addition, mechanical properties of poly(α-olefin)s were studied, among which poly(1-octene) exhibited the highest tensile stress-at-break (17.9 MPa) and strain-at-break (1480%), while poly(1-pentene) showed the best strain recovery (81%).

Dual roles of trifluoroborate in nickel-catalyzed ethylene polymerization: Electronic perturbation and anchoring for heterogenization

Brookhart-type α-diimine nickel and palladium catalysts have been extensively studied over the past several decades; however, the heterogenization of these metal complexes has received much less attention. In this contribution, we installed a trifluoroborate potassium substituent on an α-diimine framework. The ionic nature of trifluoroborate potassium endowed the α-diimine nickel complex with a strong affinity for the SiO2 support, while its electron-donating nature enhanced the catalyst stability and polyethylene molecular weight. In the presence of only 100 equiv. of Et2AlCl cocatalyst, the SiO2-supported catalyst demonstrated significantly better performance than its homogeneous analog during ethylene polymerization, with extremely high activity (1.42–6.53 × 107 g mol−1 h−1) and high thermal stability. The heterogeneous system led to the formation of high-molecular-weight polyethylenes (Mn 142,500–732,800 g/mol), narrow polydispersities (2.18–3.00), tunable branching densities (21–64 per 1000 carbon atoms), and great mechanical properties. Moreover, the efficient copolymerization of ethylene with comonomers such as methyl 10-undecenoate, 6-chloro-1-hexene or 5-hexenylacetate was achieved. These superior properties enabled by the trifluoroborate potassium moiety may inspire its applications in other polymerization catalyst systems.

Polymeric α-diimine palladium catalysts for olefin (co)polymerization

In this contribution, a series of α-diimine palladium complexes with two vinyl units were designed and synthesized. Tandem ring opening metathesis polymerization/cross metathesis of cycloolefins in the presence of these complexes as chain-transfer agents resulted in a series of well-defined polymeric α-diimine Pd catalysts with improved catalytic performances. These polymeric Pd complexes can catalyze polymerization of ethylene and result in increased molecular weight of polyethylene products compared with their molecular analogues. In the polymerization of 1-hexene, the polymeric catalysts led to less isomerization, resulting in increased conversions of 1-hexene and higher molecular weights. Moreover, polymeric palladium catalysts show improved tolerance to polar comonomers, leading to higher activities as well as higher molecular weights in ethylene–methyl acrylate copolymerization. Most importantly, tuning of catalyst performance can be achieved by varying the structure or amount of cyclic monomer used in the preparation of these well-defined polymeric catalysts starting from the simple homogeneous platform.

Rigid α-diimine palladium complexes as direct C-H arylation precatalysts for thiophenes and heteroaryl bromides

• An array of rigid α-diimine palladium complexes were reported. • These palladium complexes efficiently promoted the direct C-H arylation under mild reaction conditions. • Cross-coupling of a variety of thiophene with heteroaryl bromides afforded excellent yields and high reigioselectivity. The direct C-H heteroarylation of thiophenes, promoted by rigid α-diimine palladium complexes is described. This approach exhibits broad substrate scopes with functional group tolerant, and proceeds with the formation of regioselective products.

A Dual Steric Enhancement Strategy in α-Diimine Nickel and Palladium Catalysts for Ethylene Polymerization and Copolymerization

A Dual Steric Enhancement Strategy in α-Diimine Nickel and Palladium Catalysts for Ethylene Polymerization and Copolymerization

Investigations of ligand backbone effects on bulky diarylmethyl-based nickel(II) and palladium(II) catalyzed ethylene polymerization and copolymerization

Recently, the sterically hindered ortho-dibenzhydryl or diarylmethyl-derived (CHPh2 or CHAr2 on the N-aryl ring) α-diimine nickel(II) and palladium(II) catalysts have received much attention due to their unique catalytic properties for olefin (co)polymerization. In this work, the synthesis and characterization of bulky diarylmethyl-derived Ni(II) and Pd(II) ethylene (co)polymerization catalysts containing a series of backbone substituents (H, CH3 and acenaphthene) were described. Substituent effect of the backbone on catalyst activity and stability, polymer molecular weight, polymer branching density and polar monomer incorporation were investigated in detail. Overall, acenaphthene-based Ni3 showed the highest catalytic activity and produced the polymer with the highest molecular weight in Ni(II) system, while CH3-based Pd2 showed the best catalytic property in Pd(II) system. The H-substituted Ni(II) and Pd(II) catalyst was notable in its low activity or no activity compared to the other substituted Ni(II) and Pd(II) catalyst. Most interestingly, Pd2 demonstrated highest polymerization activities under gas-phase conditions through the self-supported catalytic mechanism.

Boosting the Thermal Stability of α-Diimine Palladium Complexes in Norbornene Polymerization from Construction of Intraligand Hydrogen Bonding and Simultaneous Increasing Axial/Equatorial Bulkiness.

Increasing the thermostability of α-diimine late-transition-metal complexes and therefore rendering them more active at higher temperatures is of great importance, yet challenging for the olefin polymerization field. In the present research, a new family of α-diimine palladium complexes that can promote norbornene polymerization at high temperatures (up to 140 °C) is disclosed. Because of the conformational restriction caused by increasing the axial and equatorial bulkiness as well as the presence of intraligand H···F hydrogen bonds, N-aryl rotations can be efficiently restricted, therefore circumventing the deactivation of the active species at high temperatures. At 80-140 °C, these complexes can efficiently catalyze norbornene homopolymerizations, giving high catalytic activities up to 5.65 × 107 g of PNB per mole Ni per hour and polymers with high molecular weights up to 37.2 × 104 g/mol, which are highly superior to catalytic systems mediated by CF3-free complexes. Moreover, these complexes could also afford medium catalytic activities in the presence of polar 5-norbornene-2-carboxylic acid methyl ester (NB-COOCH3).

Unprecedented Steric and Positioning Effects of Comonomer Substituents on α-Diimine Palladium-Catalyzed Vinyl Arene/CO Copolymerization

To better understand the substituent effects of vinyl arene, a series of substituted styrenes with different groups and locations (n-X-St, n = 2, 3, 4; X = H, Me, tBu, MeO, tBuO, F, Cl, and Br) were used as comonomers for palladium-catalyzed vinyl arene/CO copolymerization. Dibenzobarrelene-based α-diimine palladium catalyst Pd1 was capable of catalyzing alternating copolymerizations of substituted styrene comonomers and CO in a living fashion, which excluded the effect of the palladium catalyst. Electronic effects of comonomer substituents were quantitatively examined by Hammet constants (σ) of substituents and highest occupied molecular orbital (HOMO) energies of comonomers. Experimental results clearly showed that the turnover number (TON) of copolymerization, copolymer molecular weight, and stereoregularity were greatly affected by the inserted substituent. The steric effect of 4-substituents was presented and clearly proved in addition to a widely acceptable electronic effect. Strikingly, the unprecedented positioning effects of comonomer substituents were initially discovered. Whatever substituents were located on 3-position, vinyl arene/CO alternating copolymerization was greatly promoted because of the neglectable steric effect of 3-substituents.

The synergistic effect of rigid and flexible substituents on insertion polymerization with α-diimine nickel and palladium catalysts

The introduction of flexible cycloalkyl groups greatly enhanced the chain growth in the Ni(ii) catalytic system and facilitated the insertion of polar monomers in the Pd(ii) catalytic system.

Combining hydrogen bonding interactions with steric and electronic modifications for thermally robust α-diimine palladium catalysts toward ethylene (co)polymerization

Thermally robust α-diimine palladium catalysts are highly active for ethylene (co)polymerization at high temperatures by steric and electronic modifications in combination with hydrogen bonding interactions.

Mono- and Dinuclear α-Diimine Nickel(II) and Palladium(II) Complexes in C–S Cross-Coupling

The usefulness of transition metal catalytic systems in C–S cross-coupling reactions is significantly reduced by air and moisture sensitivity, as well as harsh reaction conditions. Herein, we report four highly air- and moisture-stable well-defined mononuclear and bridged dinuclear α-diimine Ni(II) and Pd(II) complexes for C–S cross-coupling. Various ligand frameworks, including acenaphthene- and iminopyridine-based ligands, were employed, and the resulting steric properties of the catalysts were evaluated and correlated with reaction outcomes. Under aerobic conditions and low temperatures, both Ni and Pd systems exhibited broader substrate scope and functional group tolerance than previously reported catalysts. Over 40 compounds were synthesized from thiols containing alkyl, benzyl, and heteroaryl groups. Also, pharmaceutically active heteroaryl moieties are incorporated from thiol and halide sources. Notably, the bridged dinuclear five-coordinate Ni complex has outperformed the remaining three mono four- or six-coordinate complexes by giving almost quantitative yields across a broad substrate scope.

Cationic α-Diimine Nickel and Palladium Complexes Incorporating Phenanthrene Substituents: Highly Active Ethylene Polymerization Catalysts and Mechanistic Studies of syn/anti Isomerization

α-Diimine palladium complexes incorporating phenanthryl- and 6,7-dimethylphenanthrylimino groups have been synthesized and characterized. The (diimine)PdMeCl complexes prepared from 2,3-butanedione...

Reversion of the chain walking ability of α-diimine nickel and palladium catalysts with bulky diarylmethyl substituents

In general, α-diimine palladium species are more likely to undergo chain walking than the corresponding nickel species, resulting in more branched and topological polyethylene. Moreover, the ligand steric effects have a significant influence on the chain walking in α-diimine system. In this contribution, a series of acenaphthene-based α-diimine ligands bearing bulky diarylmethyl moieties with various electronic effects and the corresponding Ni(II) and Pd(II) complexes were synthesized and characterized. These Ni(II) complexes exhibit high activities in ethylene polymerization even at 80 °C, generating ultrahigh-molecular-weight polyethylenes with low branching density and high melting temperature. The corresponding palladium complexes display moderate activity, leading to semicrystalline polyethylene with low branching density and high melting temperature. Polar functionalized semicrystalline polyethylene with high melting temperature can also be obtained via copolymerization of ethylene with polar monomers using these palladium complexes. Moreover, the remote nonconjugated electronic substituents exert a great influence on the ethylene (co)polymerization. Most importantly, the chain walking ability of metal species can be controlled by changing the ligand steric environment, and the diarylmethyl substituents can even reverse the chain walking trend of palladium and nickel species.